Method and system for unified mobile content protection

ABSTRACT

Media content is delivered to a variety of mobile devices in a protected manner based on client-server architecture with a symmetric (private-key) encryption scheme. A media preparation server (MPS) encrypts media content and publishes and stores it on a content delivery server (CDS), such as a server in a content distribution network (CDN). Client devices can freely obtain the media content from the CDS and can also freely distribute the media content further. They cannot, however, play the content without first obtaining a decryption key and license. Access to decryption keys is via a centralized rights manager, providing a desired level of DRM control.

This application is a Continuation Application of U.S. patent application Ser. No. 14/563,642, filed on Dec. 8, 2014, now patented under U.S. Pat. No. 9,858,396 on Jan. 2, 2018, which is a Continuation Application of U.S. patent application Ser. No. 13/370,537, filed Feb. 10, 2012, which is a Continuation Application of International Application No. PCT/US2010/045596, filed Aug. 6, 2010, which claims priority to U.S. Provisional Patent Application No. 61/234,094, filed Aug. 14, 2009, the disclosure of which are fully incorporated herein by references.

BACKGROUND

The growing number of large form factor mobile devices such as the iPad has revolutionized mobile media consumption leading to revolutionary initiatives such as “TV Everywhere™” (TVE) with a mandate to make premium content available on a wide range of devices with great diversity in capabilities. This type of distribution, sometimes known as “Over-The-Top” (OTT) distribution, has underscored the need for a new and more robust trust model that builds on a 2-part trust model of user authentication and device identification and can offer the same level of content protection that content owners have had in the closed Consumer Electronics ecosystems of the past. The added level of protection can enable publishers to fully realize the potential for content distribution through this new open ecosystem of devices.

Content protection is challenging in mobile devices for a number of reasons. Mobile devices do not uniformly support Digital Rights Management (DRM) standards. In particular, most mobile devices do not currently support the most comprehensive form of content protection, the Open Mobile Alliance (OMA) V2.0 DRM standard. Mobile devices also vary in their CPU performance and memory capacity. An additional complication is the need to support multiple modes of delivery required in the mobile environment, such as live streaming, watching short video segments, rentals, or media download for watching later.

Current media protection schemes depend on sending the license information in-band with the media or using a pre-distributed license key in the media viewing device. Examples are Playready, WMDRM, Widevine, and Flash Access. However, TVE requires that the rights are transferable across devices in a seamless manner.

SUMMARY

The present invention relates in general to protecting media on mobile devices and more specifically to implementing a content protection system for media that may be streamed or watched offline on mobile devices. This system is particularly useful in the deployment of TVE services for protecting media on any Internet-connected device in an “Over-The-Top” (OTT) manner where the digital rights to the media are delivered to the device over the network and made specific to the device and user.

Methods and apparatus are disclosed for protecting content delivered to a variety of mobile devices based on client-server architecture with a symmetric (private-key) encryption scheme. In one embodiment, a media preparation server (MPS) encrypts all media content and publishes and stores it on a content delivery server (CDS), such as a server in a content distribution network (CDN). Clients can freely obtain media content from the CDS and can also freely distribute it further. They cannot, however, play the content without first obtaining a decryption key. Access to decryption keys is via a centralized rights manager, providing a desired level of DRM control.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention.

FIGS. 1 and 2 are block diagrams of systems capable of conducting procedures, in accordance with various embodiments of the invention;

FIG. 3 is a diagram of the content encryption, in accordance with an embodiment of the present invention;

FIG. 4 is a diagram of the key wrapping, in accordance with an embodiment of the present invention; and

FIG. 5 is a block diagram of a system showing interfaces and message formats between system components;

FIGS. 6-8 are diagrams of message flows during system operation in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram for one embodiment of the present invention. It shows a client device 10 and a plurality of servers 12 that are connected together in a secure network and together form an instance of what is referred to herein as a “wireless platform” (WP). The client device 10 and WP servers 12 are typically computerized devices which include one or more processors, memory, storage (e.g., magnetic or flash memory storage), and input/output circuitry all coupled together by one or more data buses, along with program instructions which are executed by the processor out of the memory to perform certain functions which are described herein. Part or all of the functions may be depicted by corresponding blocks in the drawings, and these should be understood to cover a computerized device programmed to perform the identified function.

In one embodiment, the servers 12 (referred to as servers herein) may be collocated in a single data center. In another embodiment, the servers 12 may be geographically distributed in multiple data centers. In another embodiment, the servers 12 may be physically in the same region, but connected to the client 10 through separate network paths (e.g. through different network service providers). In one embodiment, the servers 12 are situated as part of a content delivery network (CDN) 14. In one embodiment, the content from a content publisher 16 is ingested via an ingestion engine 18, and the ingested content is then segmented by a media preparation engine (MEDIA PREP) 20. The media preparation engine 20 obtains a content encryption/decryption key from a digital rights management (DRM) server 22 and uses it to encrypt the content for storage and later delivery in encrypted form. An example of streaming of content is shown in published PCT application WO 2010/045109.

In part of the description below, the combination of the ingestion engine 18, media prep 20 and a play manager 26 are referred to as a “content controller”. Thus in one embodiment the system is constituted by a content controller along with a DRM server 22 and a rights manager 24.

A media preparation profile in the media preparation server 20 specifies an encryption type on a per-media-item basis. Candidate ciphers may include XOR, RC4, HC-128, AES, and along with the specification of encryption type is stored a corresponding key and a key length. Each user-selected media item has its own randomly generated key value. In the case of AES and XOR encryption, this randomly generated key value is used as the actual key for encryption/decryption, whereas for RC4 and HC-128 it is the seed key to initialize a stream cipher. AES key length is typically 128 bits. XOR key length may be 1024 bytes, and may be configurable. RC4 and HC-128 use 128 bit seed keys. The media preparation profile also specifies, on a per-media basis, the length of the byte stream which should be generated (this is the actual key used for media encryption, and the length is the same as the block length described elsewhere herein). Each user-selected media item is transcoded in multiple formats for different target platforms, and each of the resulting transcoded media files is immediately encrypted with the chosen cipher and key and the encrypted files are then pushed to the CDN 14. Additional details regarding encryption are provided below.

In order to use the system for downloading content, the client device 10 first authenticates with the rights manager 24 and registers its device with the DRM server 20. During this process the client 10 obtains a logical device id from the rights manager 24 that is a token to represent a user of the client device 10, and associates this token with the specific client device 10 via a device “fingerprint” which is a unique identifier for the client device 10. The unique identification may be based on certain physical properties that may include an international mobile equipment identifier (IMEI) number, media access control (MAC) address, or certain file system properties. Each of the supported client devices 10 provides an Application Programming Interface (API) via which the unique identifier of that device can be obtained. Some devices have an IMEI number, some a mobile equipment identifier (MEID) number, some an electronic serial number (ESN). The iPhone has a unique device identifier (UDID).

The client device 10 has a built-in domain key that is used to encrypt the exchange of the logical device ID with the rights manager 24. For enhanced security, the domain key is divided into a number of separate components which are stored so as to be difficult to locate. For example, each may be stored as an array, with elements of the array represented as strings containing an integer and some special characters which are commonly found in binary files. When the arrays are examined, it is difficult to detect where the components are located. At run-time, all arrays are processed, special characters are discarded, and elements of these arrays are converted into characters and concatenated together to produce the actual domain key.

Device registration is carried out as follows. A DRM Agent running on the client 10 generates an encrypted token containing a device id and a randomly generated long nonce. That information (device id and the random long nonce) is encrypted with the domain key and is sent to the DRM server 22. The DRM server 22 decrypts this registration message using the same domain key, and stores an association between this user, device id, and key nonce in a database. The DRM server 22 generates a response containing a unique logical id assigned to this user. This response is encrypted with a session key constructed from domain key, device id, and the random key nonce provided by the DRM Agent, and the encrypted response is sent to the client 10. Details regarding the construction of the session key are provided below.

Once the client 10 receives the response, it decrypts the response and stores registration information into an encrypted rights file on the client device 10. The rights file is encrypted with the key constructed from the combination of the domain key and the device id.

The session key may be constructed as follows:

A shared or domain key is combined with the device id and the randomly generated key nonce: shared_key+device_id+key_nonce. The resulting string is fairly long, so a hash or checksum is computed on it. In one embodiment, a hex representation of the hash, which may be 32 bytes long, is chosen to be the key. In different embodiments, the raw hash output (which may be a 128-bit integer) may be used. In one embodiment a Message Digest 5 (MD5) hash may be used. Other embodiments might use a 64-bit RACE Integrity Primitives Evaluation Message Digest (RIPEMD) hash function instead of MD5.

In other embodiments, it is possible to include other individualization parameters into these keys. Thus, the client/server session key, as well as the key used to encrypt the rights file on the device, could be enhanced further by adding unique user information (user token) and/or application information, such as the application name or identifier. That way, keys will be application-specific and user-specific as well as device-specific.

FIG. 2 shows a slightly different embodiment of the system, in which there is a connection directly between the rights manager 24 and the DRM server 22 to enable the DRM server 22 to directly consult with the rights manager 24 as may be required.

As previously mentioned, any of various content encryption schemes may be employed. The following presents several specific examples along with corresponding details regarding how encryption/decryption is carried out. In some embodiments, the encryption can be applied to portions of the file such as key frames for video in order to reduce processing load.

XOR

In one embodiment the following simple and fast symmetric (private key) encryption scheme is used. Operation is illustrated in FIG. 3. The media preparation server 20 performs an exclusive-OR operation (XOR) between the contents of the media file and a secret (private) key K1 (not shown). In one embodiment the key K1 is 1024 bytes long. The XOR operation starts at a random position P1 within the media file and continues until the end of the file. The random position P1 is preferably chosen to be close to the beginning of the file, e.g. within the first 10% of the file. P1 can also be a predetermined fixed position, for example the very beginning of the file (location 0).

The key K1 may be chosen in a variety of ways. For example, it may be generated randomly. Alternatively, it may be generated by first choosing another random position P2 (not shown) within the same file, and selecting 1024 bytes from the media file starting at position P2. If there are not 1024 bytes remaining between P2 and the end of the file, then 1024 bytes are selected starting at position P2−1024+1. As noted, the key length may be other than 1024 bytes, and may be configurable.

The media preparation server 20 stores P1 and P2 in a database for each media file. In addition, the media preparation server 20 associates an expiration time with the encryption keys, stores the expiration time in the database, and re-encrypts content with new keys upon key expiration.

RC4-Drop(n)

RC4-drop(n) is a stream-cipher algorithm generally known to those skilled in the art. It includes the dropping of the first 3072 bytes from each generated keystream. Also, RC4 does not have a formal notion of an initialization vector (IV). Instead, a checksum is computed on a concatenated key and an arbitrarily chosen initialization value, and the checksum is used as the key.

In one embodiment of stream cipher encoding, the entire media file is divided into smaller blocks of a selected block size. With a stream cipher, one can generate an infinitely-long stream of bytes. Theoretically, if a content item (e.g., movie) were to be played only from start to finish, without rewinding or fast-forwarding (i.e. without scrubbing), a stream cipher could be used on the streaming media without specialization. However, since the user may scrub during playback, decryption requires a modification to the stream cipher. The media is divided into fixed-size blocks and a new stream of key bytes is generated for each block by using the same seed key and a different IV. The IV in this case can be just the sequential block number, starting from 0. In one embodiment the blocks can have length 32 k, but the block length can be different in other embodiments and may be configurable.

HC-128

HC-128 is another well-known stream cipher whose block size can be adapted as described above. Also, in addition to block size, both RC4 and HC-128 can take into account a segment number for live streaming and for video on demand (VOD). The entire long-form content is represented as many segments, and each segment is then divided into multiple blocks from the encryption/decryption point of view.

AES

The same approach to block sizing may be taken for AES unless of course in some embodiments the decryption is done in hardware. It may be desirable to use the same form of AES encryption supported by iPhone® and iPad®, which is AES bit with Cipher-Block-Chaining (CBC mode). Each segment is encrypted individually, and the same key is used across all segments, but each segment has its own initialization vector which is the sequence number of the segment.

It is briefly described how a user obtains a rights object (RO) to use in downloading and streaming, as well as playing, content. The user registers with a content provider using, in one embodiment, OpenID technology and obtains a user token which uniquely identifies that user. Before the user can play a given media content file, the user must obtain the decryption key. A DRM agent running on the client device 10 contacts the rights manager 24 and provides three items: <device-id, media-id, user-token>, where device-id is a unique identifier specific to that particular mobile device, media-id is a unique identifier specific to the particular media content the user wants to play, and user-token is the unique user identifier. Device id could be the unique address of the mobile device, or it may be one of the types of device identifiers discussed above.

The rights manager 24 receives the request for the RO from the client 10, containing <device-id, media-id, user-token>. The rights manager 24 validates the user-token using OpenID technology and also validates that media-id is correct and has not expired. It then generates the requested RO, which contains a key value K1 for media content decryption, a remaining play count for that media, and a media license expiration time. Even though communications between the client 10 and rights manager 24 is carried over a secure connection (SSL), the rights manager 24 may optionally encrypt the RO so that encrypted RO can be safely stored on the client device 10.

The encryption of the RO is illustrated in FIG. 4. To encrypt the RO, the rights manager 24 uses the following symmetric encryption scheme. The RO is encrypted with 64-bit Blowfish key constructed from the checksum (domain key+device id+key nonce). To compute K2, the rights manager 24 applies an MD5 checksum function to the device-id.

Message Flow

FIG. 5 contains a block diagram showing interfaces (numbered reference points) between system components. The following is a description of messaging flows (including message formats) among the components.

The ingestion flow consists of secure transfer of content from the content publisher 16 via a secure transfer method such as scp, sftp, or ascp (Aspera) to the content controller back-end server 28, which in turn transcodes and encrypts the content using the chosen content cipher (e.g., AES or HC-128) and publishes it into the CDN 14.

The interfaces and associated protocols are described next for each of the numbered reference points in FIG. 5.

1. Over this HTTP interface, the client 10 performs a one-time device registration with the rights manager 24 (and DRM Server 22) passing the device-id, key nonce, and message nonce encrypted with the Blowfish algorithm using the domain key that is stored in an obfuscated manner in the application binary as described above. The registration information is passed through to the DRM Server 22 via the interface 2 described below. Depending on specific deployment requirements, the client may 10 may alternatively go to the DRM server 22 first, and the DRM server 22 then communicates with the rights manager 24.

Also, on the same interface, every time the client 10 needs to play a media, it sends media rights requests to the rights manager 24 also encrypted via Blowfish with a device-specific key. The media rights request contains device id, media id, logical id (a unique abstract user identifier) provided by the DRM server 24 when the device was registered, message nonce, and the current play count.

2. This HTTP interface is used as a pass-through interface, where the rights manager 24 relays requests (device registration and media location and rights requests) received from the client 10 and destined to the DRM server 22. These messages are encrypted as noted in #1. The rights manager 24 maintains user information which the DRM server 22 does not have access to, and the rights manager 24 maps individual users to logical ids maintained by the DRM server 22. The rights manager 24 appends the logical id, uniquely identifying the current user, to all requests being forwarded to the DRM server 22. The only exception is the initial device registration because it does not have a logical id for that user at that point. The logical ids need not be encrypted when these servers are in a secure facility 30 with restricted access as shown. In environments where these servers need to be remote, a secure connection would be needed between them. The secure connection may take the form of a virtual private network (VPN) or a Secure Sockets Layer (SSL) connection.

3. This HTTP interface is used by the DRM server 22 to request media information from the back-end content controller 28. This interface is used to obtain information needed to play a user-selected media item. The request by itself does not have any commercial value and is therefore not encrypted nor sent over a secure channel.

4. This HTTP interface carries the response of the content controller 26 to the DRM server request described under item #3 above. The response is an XML document, containing media URL pointing to an encrypted media file located in the CDN 14 and an encrypted message which contains information about the cipher and the key used to encrypt this media. The message is encrypted with the Blowfish algorithm and the domain key.

5. Via this HTTP interface the DRM server 22 asks the rights manager 24 for media rights for the current user. The request contains logical id, media id, and the play count reported by the client 10. This interface is used when the client 10 is requesting media rights as described in #1. The information need not be encrypted when the DRM server 22 and rights manager 24 are in a secure facility 30. Alternatively, a secure connection may be employed.

6. This HTTP interface carries the response of rights manager 24 to the DRM server request described under item #5 above. The response is an XML document containing rights information for the requested media and the current user. This interface is used only when the client 10 is requesting media rights. The response need not be encrypted when the DRM server 22 and rights manager 24 are in a secure facility 30. Alternatively, a secure connection may be employed.

7. This HTTP interface sends the response of the DRM server 22 to the rights manager 24. Two types of responses are sent over this interface: the device registration response and the media location and rights in response to requests described under item #2 above.

The device registration response is an XML document that contains an encrypted message (containing the logical id) destined for the client 10, and also the logical id and total device count for the current user in the clear. The rights manager 24 uses the device count to check against the total count of authorized devices for the user. It removes the logical id and device count from the response, before forwarding it to the client 10 on interface 8. The client completes the registration on its end when it can receive the encrypted message and successfully decrypt and verify the nonce and checksum in the message.

The media rights and location response is an XML document that contains the media URL pointing to an encrypted media file located in a CDN 14, and an encrypted message (destined for the client) which contains information about the cipher and the key needed to decrypt this media and media rights information for the current user. This response is forwarded to the client 10.

In both types of responses, the message is encrypted with a key produced from the domain key, device id, and the key nonce.

8. This is a pass-through interface where the rights manager 24 simply forwards the responses it received from the DRM server 22 to the client 10, in response to the client's requests described under item #1 above. The contents of these responses are described fully in #7.

9. This is the interface by which the content is delivered to the client 10 from the CDN 14.

FIG. 6 is a message flow ladder-diagram for one embodiment of the present invention. It describes the message flow for device registration and obtaining the rights object containing the content key for playing the content. FIG. 7 contains another message flow ladder-diagram for an alternate embodiment where the client 10 is in direct communication with the intervening rights manager 24, which in turn communicates with DRM server 22.

FIG. 8 illustrates a work flow for integrating functionality of a certificate authority (CA) into the content protection scheme. The work flow consists of the following steps:

1. Have a server certificate signed by the CA

2. Distribute the application to devices via application stores

3. Initially, a client 10 authenticates with a server via SSL via the following: [0056] A. Authentication could be passed through to a customer authentication server 12 [0057] B. The customer authentication server 12 receives an activation code from the CA and passes it to the client 10 [0058] C. The client 10 uses the activation code (using tools of a software development kit (SDK) of the CA) to obtain an encrypted security credential from the CA, wherein the security credential=(the shared secret, a credential ID, and a creation time) [0059] D. The client 10 sends the credential ID to the server 12 which is linked to an authentication record [0060] E. The client 10 registers by sending the device fingerprint to the server 12 and gets a device-specific key

4. For a session, a client 10 is validated as follows: [0062] A. The client uses the CA SDK to dynamically generate a security code from the security credential and sends it to the server 12 via SSL [0063] B. The server 12 contacts the CA to validate the client 10 using the stored credential ID together with the security code [0064] C. The server 12 returns the content key encrypted with the device-specific key [0065] D. In offline mode, the DRM agent of the client 10 will offer protection with an offline timeout that forces contact with server 12 (which includes protection against clock tampering as described below)

Anti-Clock Rollback Protection

Clock rollback is a technique employed to illegally extend time-based licenses. A user manipulates the clock on a playback device so that the time-based license expiration is reached later than it should (or not at all). To detect clock roll-back, time is sampled on the client device 10 when the application is registered and every time it starts up, and the time is stored into the encrypted file. When a player is instantiated to play a user-selected media item, a separate thread is also started to monitor the progression of time during playback. The thread sleeps for a short time period, wakes up, and increments an elapsed time counter. That elapsed time is added to the last known local time. Thus, the application always has information about what the time should be (to an approximation). This technique can be augmented to include time information from a server 12.

Rights File Integrity Protection

The rights file (also referred to as rights object herein) is stored on the client device 10 and contains the device-specific key and the content-keys encrypted with the device-specific key. The rights file itself is encrypted with the key constructed from the domain key and the unique identifier of the device. The contents of the file are checksummed and the checksum itself is stored within the file. When the file is decrypted, the contents are checksummed again and the computed checksum is compared with the checksum stored in the file to verify that the file has not been tempered with. The rights file also has a copy protection feature, in a sense that an outdated copy of the file cannot be written over the fresh copy without being detected by the DRM Agent. The copy protection is platform-dependent. On the iPhone/iPad platforms, DRM Agent obtains a unique property of the file and stores it within the encrypted file. The unique file value is not something that can be controlled at will, it is a property that is assigned by the operating system. Those skilled in the art may choose this file property such that copying the file would force a change in the unique value. On Android the rights file is stored within the application-specific directory which is protected from other applications and from user access via standard Linux permissions. Furthermore, DRM Agent generates a random long number and stores it within the encrypted file as well as within the application-specific directory on the device. The two numbers are compared when mobile application starts. On the Blackberry platform, a similar randomly generated long number is stored inside the encrypted file as well as within the application-specific persistent secure storage offered by the Blackberry platform.

In the description herein for embodiments of the present invention, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention.

Although the above description includes numerous specifics in the interest of a fully enabling teaching, it will be appreciated that the present invention can be realized in a variety of other manners and encompasses all implementations falling within the scope of the claims herein. 

What is claimed is:
 1. A client device for obtaining and playing protected content on the client device, comprising: at least one processor; a computer-readable storage medium, coupled to the at least one processor, the computer-readable storage medium including computer-readable instructions, when executed by the at least one processor, are configured to: register the client device by engaging in a device registration process with a digital rights management (DRM) server, the registration process including sending device information encrypted with a secret domain key built into the device to the DRM server and establishing a device-specific secure communications channel with the DRM server as well as a device-specific rights encryption key, the device-specific rights encryption key being shared with the DRM server and generated using the device information and the domain key; obtain a license for use of the protected content by sending a media rights request to the DRM server via the device-specific secure communications channel and receiving a media rights object via the device-specific secure communications channel in response to the media rights request, the media rights request being encrypted with the domain key, the media rights object being encrypted with the device-specific rights encryption key and including media location information and a media decryption key, the media location information identifying a location in a content distribution network from which the protected content is to be downloaded for playback, and the media decryption key being usable by the client device to decrypt the encrypted media item upon being downloaded by the client device; decrypt the received media rights object using the device-specific rights encryption key to access the media location information and media decryption key; obtain the protected content from the content distribution network using the media location information and decrypting the protected content using the media decryption key; and play the decrypted content.
 2. The client device of claim 1, wherein the computer-readable instructions are further configured to detect clock roll-back by use of a DRM agent executing on the client device in order to: monitor a local clock to ensure that a time reported by the local clock is continuously incrementing; at a time of starting a playback application, sample the time and comparing the time to a last known time stored in an encrypted rights file on the client device; and employ a separate thread which wakes up at regular intervals as playback proceeds and (1) keeps track of elapsed playback time, (2) adding the elapsed playback time to the previously sampled local time on the client device, and (3) writing the result to the encrypted file.
 3. The client device of claim 1, wherein the computer-readable instructions are further configured to detect content expiration by: maintaining rights information including an expiration time and/or maximum play count in association with each media item, the right information being received from a digital rights management (DRM) server in an encrypted form, and storing the rights information in an encrypted file on the client device; and upon initiating playback of the media item, employing a DRM agent on the client device to determine whether the media item has expired by comparing a current time on the client device to the expiration time of the media, and/or comparing the maximum play count to a stored actual play count, the DRM Agent ensuring that clock rollbacks on the client device are detected and disallowing media playback if time is not steadily incrementing, the DRM Agent executing a separate thread, in parallel with a thread performing playback such that the media item can expire not only upon initiating playback but also during playback.
 4. The client device of claim 1, wherein the device registration process includes establishing device specificity by: uniquely identifying the client device using a unique physical device identifier, the unique physical device identifier being encrypted with a domain secret and stored in an encrypted file on the client device and also sent to a digital rights management (DRM) server via an encrypted message, the encrypted unique physical device identifier being stored by the DRM server in a database of registered client devices and being mapped to a unique logical device identifier representing a user of the client device, the unique logical device identifier being sent encrypted with the domain secret.
 5. The client device of claim 4, wherein the unique physical device identifier is selected from a group consisting of an IMEI (international mobile equipment identifier) number, an MEID (mobile equipment identifier), an ESN (electronic serial number) and a UDID (unique device identifier).
 6. The client device of claim 4, wherein the computer-readable instructions are further configured to: combine the unique physical device identifier with the domain key to generate the device-specific rights encryption key.
 7. The client device of claim 1, wherein the device-specific rights encryption key includes additional individualization parameters beyond an identification of the mobile client device.
 8. The client device of claim 7, wherein the other individualization parameters include unique user information and/or unique application information for an application program executing on the client device and containing the DRM agent.
 9. The client device of claim 8, wherein the unique application information includes an application name or identifier. 